This invention relates to polymeric bodies having a multilayered structure, and more particularly relates to multilayered thermoplastic tubular structures prepared by coextrusion techniques having a specifically positioned low gas permeable layer therein, said structures being suitable for producing articles including containers and the like.
The formation of multilayered structures is known wherein the separate layers contribute to the final properties. Many of these composite structures may be readily prepared by melting each individual component in separate zones and subsequently combining them in a predetermined configuration. In general, composite structures of two or more different polymeric materials may be readily coextruded to form a multilayered structure. In particular, a multilayered tubular structure may be readily made by conventional coextrusion processes wherein each thermoplastic material which is ultimately to form a layer in the composite is separately heated to its melt extrusion temperature by a conventional melt extruder and thereafter each melted material is forced by pressure feeding means into streams of melted thermoplastic material that enter a multimanifold die device provided with toroidal chambers and a common annular die orifice. The melted materials are fed into said device and form therein continuous annular layers as they exit the die orifice, each additional layer being fed to the device and issuing as annular coaxial layers that are essentially concentric with the initially formed annular layer. In effect, the device provides an annular flow of plastic materials, one strata of thermoplastic material being circumferentially deposited upon the preceding one to produce the multilayered tubular body.
In distributing thermoplastic materials into a multimanifold die device it has been found that several polymeric materials have the tendency to decompose or degrade when subjected to certain elevated temperatures, the elevated temperatures being reached oftentimes by heat from an adjacent coextruded material that must be processed at a higher melt extrusion temperature. Several polymers exhibit this tendency and, especially prone are the so-called low gas permeable polymers. These rather susceptible polymeric materials when subjected to elevated temperature have the propensity to partially degrade or decompose whereby unwanted gaseous products or inclusions are formed that become entrapped and are embedded in the polymeric interlayers or matrix. These inclusions often serve as nuclei or sites of buildup for the ultimate deposition of various organic and inorganic substances in a finished article. In particular, this deposition is noticeable with tubular forms and, especially, when they are further processed via blow-molding to form necked containers. It should be mentioned that in addition to the degradation or decomposition of these susceptible polymers there are apparently other phenomena at work that accentuate or cause a greater likelihood for this condition to exist, not only in frequency but in magnitude. Since there are a number of different polymeric materials that may be coextruded at the same time to form a given tubular body there is a marked tendency during any subsequent quenching or cooling for the various cojoined materials to contract or shrink at different levels resulting in stresses within the tubular structure. These stresses appear somehow to accelerate the formation of gaseous inclusions or bubbles that apparently display themselves. Seemingly, the differential contraction between the faster cooling outer layer where generally less shrinkage occurs and the slower cooling inner layer where generally more shrinkage occurs accelerates the buildup of these inclusions.
Where potentially degradable polymeric materials are incorporated as a layer or layers in tubular structures in accordance with this invention there is a marked improvement in properties advantageous to further fabrication including a substantial reduction in the tendency to form organic or inorganic inclusions. In essence, the multilayered tubular structure of the subject invention is novel and provides for unique properties and improvements not only in gas permeability but also in the mechanical properties of containers formed from said tubular structures including bottom wall sealability and neck configuration, which in turn, enables them to be useful for many purposes in the art, especially for parisons in blow molding containers as hereinafter set forth.
Although the particular problems resolved by the subject invention have not been appreciated or squarely addressed by the prior art, there are assuredly related and pertinent patents in this field. As a brief overview, a number of patents disclose methods of making tubular, multilayered structures as well as methods of blow modling certain thermoplastic structures. For example, the method of coextruding multilayered tubular bodies is described in U.S. Pat. No. 3,308,508 to Schrenk as well as U.S. Pat. No. 3,354,506 to Raley; multilayered, flexible structures having at least one layer of low gas permeability are described in U.S. Pat. No. 3,570,748 to Coyle, and U.S. Pat. No. 3,956,544 to Harrington as well as U.S. Pat. No. 4,139,665 to Herrero; and also rigid multilayered sheet structures are shown in U.S. Pat. No. 3,645,838 to Newman, et al., and U.S. Pat. No. 4,182,457 to Yamada, et al.; and a useful process for blow molding tubular bodies or parisons into high strength transparent hollow objects including containers is described in U.S. Reissue Pat. No. 26,956 to Wiley.